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Bivalves
are a class of molluscs that possess a shell made up of two halves
(the right and left valve). As the body is completely enclosed in
the shell, evolution has led to the loss of the head
The
majority of bivalves are filter feeders, large quantities of water
are pumped through the shellfish and food particles are ensnared
by filaments and passed to the mouth. Almost all of the other bivalves
are deposit feeders. These have a syphon through which water is
“sucked through”, before the food particles are sorted and passed
to the mouth, this allows the shell to be completely covered by
sand with only the tip of the syphon protruding. The rest a small
number feed differently, such as the shipworm Teredo which eats wood
as it bores.
There
are four type of life strategies used by bivalves; sessile, burrowing,
boring or free moving.
Sessile
molluscs fix themselves to a hard surface and remain attached for
their lifespan. This allowed molluscs to colonise areas of rock
where it was impossible to burrow. Fixing can be by byssal threads
or cementation. Byssal threads are tough chitinous threads secreted
by the byssus gland in the foot, as is the case with mussels. Molluscs
like the oysters cement one of their valves directly to the rock
surface.
There
are sand burrowing molluscs that are almost sessile in nature. These
are not attached to anything but due to their physiology are “stuck”
in their burrows. Species like the Sand Gaper Mya arenaria have
a feeding siphon that cannot be retracted into its shell plus the
foot is inadequate for burrowing.
There
are species that bore into soft rocks such as the Common Piddock
Pholas dactylus and the shipworm Teredo which bores into wood.
The
most popular life strategy is to be free moving. The majority relying
on their muscular foot to burrow through the sand. The mud is thrust
foreward and backwards and at the same time by the opening and closing
of the valves expelling water into the sand and so fluidising the
sand, easing movement through it. The expelling of water can be so
strong that species, such as the scallops use it as a method of movement,
this allowing propulsion through water.
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